Linear Compressor And Corresponding Drive Unit

ABSTRACT

A drive unit for a linear compressor comprising a frame and an oscillating body. The oscillating body is mounted in the frame via at least one diaphragm spring and can be moved back and forth in one direction. The diaphragm spring comprises a plurality of limbs, fastened with one end to the frame and with the other end to the oscillating body, which are associated with respective readjusting springs that counteract a deformation of the arm.

This invention relates to a linear compressor, in particular for use for compressing refrigerant in a refrigerating device, and in particular a drive unit for driving an oscillating piston movement for such a linear compressor.

U.S. Pat. No. 6,506,032B2 discloses a linear compressor whose drive unit comprises a frame and an oscillating body mounted in the frame by means of a diaphragm spring. The oscillating body comprises a permanent magnet, a piston rod rigidly connected to the permanent magnet and a piston articulated to the piston rod, which piston moves back and forth in a cylinder. The movement of the piston is driven by an electromagnet arranged around the cylinder, which electromagnet interacts with the permanent magnet. A disc-shaped diaphragm spring is screwed centrally to the piston rod and the outer edge of the diaphragm spring is connected to a yoke which surrounds the cylinder, the electromagnet and the permanent magnet.

The oscillating body and the diaphragm spring form an oscillating system whose natural frequency is determined by the mass of the oscillating body and the diaphragm spring, as well as by the stiffness of the diaphragm spring. The diagram spring only permits small oscillation amplitudes because any deflection of the oscillating body is associated with an expansion of the diaphragm spring. Due to the low oscillating amplitude it is difficult to reduce the dead volume of the cylinder reliably. However, the higher the dead volume the lower the efficiency of the compressor. The short stroke also necessitates designing the cylinder with a diameter that is proportional to the length in order to achieve a given throughput. It is expensive to seal the correspondingly large circumference of the piston sufficiently.

Since the oscillating body is only retained in the radial direction by its connection to the spring, it is possible that the head of the piston rod supporting the piston may oscillate back and forth and grind against the cylinder wall. To prevent this a compressed gas bearing is provided for the piston, i.e. the cylinder wall covered by the piston has openings which are connected to the high pressure outlet of the linear compressor to form a gas cushion between the inner wall of the cylinder and the piston. However, such a compressed gas bearing only functions if the required excess pressure is present at the outlet of the linear compressor, i.e. not when the compressor starts or stops. At these times there is a risk that the piston will grind against the cylinder wall, resulting in premature wear of the compressor.

A linear compressor is disclosed in U.S. Pat. No. 6,641,377 B2. In this double-piston linear compressor each piston is retained by two two-armed diaphragm springs.

Due to the curvature of the limbs a longer piston stroke is possible. The limbs are more easily deformable, in the longitudinal direction of the piston, than transversely to it, so that they counteract a contact between the piston and the cylinder wall.

To achieve a desired throughout of the compressor the oscillating frequency of the piston must not be too low. This oscillating frequency is all the higher the stiffer the diaphragm spring. However, there is a risk that too rigid a diaphragm spring may result in fatigue at high oscillation amplitudes.

The object of this invention is to provide a drive unit for a linear compressor with a frame and an oscillating body mounted by means of a diaphragm spring, in which the diaphragm spring permits a long stroke of the oscillating body without risk of fatigue and which is able to achieve a high throughput with a small piston diameter.

To achieve a long stroke without the risk of material fatigue, the limbs of the at least one diaphragm spring should be produced from a very thin material. Its strength may be just sufficient to prevent lateral deflection of the oscillating body. However, such a weak diaphragm spring would result in a low natural frequency of the drive unit and hence, at a predetermined stroke, in a low throughput of a compressor driven by the drive unit. In order to achieve a natural frequency of the drive unit adequate for a required throughput, a readjusting spring is therefore assigned, according to the invention, to each limb, which spring counteracts a deformation of the limb so that the diaphragm spring, together with the readjusting springs, forms an elastic system whose stiffness is considerably greater than that of the diaphragm spring alone.

In the simplest case each limb has an individual section curved in one direction. Each such limb also exerts a torque on the oscillating body supported by it when deflected, so that together with the back and forth movement a rotary oscillation of the oscillating body is also excited. To prevent such a rotary oscillation from having a disturbing effect, a rotationally symmetrical structure of at least parts of the compressor may be required.

However, pairs of limbs curved in opposite directions may also be provided. In such a structure the torques induced on the differently curved limbs are mutually compensating, so that the oscillating body performs absolutely no or hardly any rotary oscillation in connection with its back and forth movement.

Each limb preferably has two sections curved in different directions. Since the differently curved sections also generate torques in opposite directions in this case too, the torque of each individual limb may therefore be made very small or caused to disappear altogether.

It is also advantageous to provide at least a second diaphragm spring whose limbs engage on a region of the oscillating body which is distant from the region of engagement of the first diaphragm spring in the direction of the oscillating movement. The oscillating body is reliably guided linearly in the direction of the desired oscillating movement by the two diaphragm springs, and a lateral deflection movement, which could result in contact between a piston supported by the oscillating body and a cylinder surrounding the piston, can be avoided.

The limbs of the same diaphragm spring are preferably joined integrally together at their ends engaging on the frame and/or at their ends engaging on the oscillating body. The ends engaging on the frame may also be connected by a frame integral with the leaf springs.

The effective spring constant of the combination of diaphragm and readjusting spring may be made adjustable so that the natural frequency of the drive unit can be adapted as required.

A helical spring is preferably used as the readjusting spring.

A further subject matter of the invention is a linear compressor with a working chamber, a piston that can be moved back and forth in the working chamber to compress a working fluid, and a drive unit of the type described above, coupled to the piston, for driving the back and forth movement.

Further features and advantages of the invention are evident from the following description of exemplary embodiments with reference to the attached figures.

FIG. 1 shows a diagrammatic section through a linear compressor;

FIG. 2 shows an elevation of a diaphragm spring for use in the linear compressor in FIG. 1 according to the invention; and

FIG. 3 shows an elevation of a second design of a diaphragm spring.

FIG. 1 shows a partially cut side view of a linear compressor. The compressor has a frame with a central chamber 21, in which openings are formed in two opposing walls, here with reference to the representation in the figure designed as ceiling 22 and floor 23, for the sake of clarity, through which openings a rod-shaped oscillating mass 24 extends with a certain clearance. Chamber 21 is provided to accommodate electromagnets, not shown, for driving a back and forth movement of a permanent magnet inserted in oscillating mass 24.

The ends of oscillating mass 24 are fastened to central regions 16 of two diaphragm springs 8 of by means of screws or rivets 25

One of diaphragm springs 8 is shown in elevation in FIG. 2. Diaphragm spring 8 has a closed outer ring or frame 13 which is rectangular in shape, which is stabilised before installation in the compressor and protects against distortion. Four limbs 14 extend from the corners of frame 13 towards central region 16, each of them being formed from three rectilinear sections 17 and two curved sections 18, 19 connecting sections 17. The two sections 18, 19 of each limb 14 are each curved in opposite directions. Four bores 20 for fastening the diaphragm spring are located in the corners of frame 13.

Frame 13 of each diaphragm spring 8 rests on bridges 26 projecting from ceiling 22 or floor 23 of central chamber 21. Diaphragm springs 8 are retained on bridges 26 by means of screws or rivets 27, which each intersect a foot section 28 of the upper and lower yoke 29, 30, respectively, and one of bores 20 in the corners of frame 13 and engage in central chamber 21. The height of bridges 26 determines the maximum stroke of the movement of oscillating mass 24; if this maximum stroke is exceeded, the central regions 16 of diaphragm spring 8 strike against ceiling 22 and bottom 23 respectively.

When central region 16 is deflected, this results in slight upward bending of curved sections 18, 19. Because of the opposite directions of curvature of the two sections 18, 19 of each limb, the upward bending gives rise to opposing torques, so that the torque exerted by each individual limb 14 on central region 16 is small. Moreover, the torques of adjacent limbs 14 are mutually compensating because each of them is the mirror image of the other and the torques exerted by them are therefore inversely the same. Central area 16, and consequently also a piston rod 10 fastened to it, are therefore guided exactly linearly and free from distortion.

Lower yoke 30 supports two helical springs 31, each of which is positioned so that free head piece 32 of these springs each touch curved sections 18 of two limbs 14, as also denoted as a dash-dot outline in FIG. 2, when they are deflected downwards and therefore resist a downward deflection of oscillating mass 24. Corresponding helical springs 31, which touch curved sections 18 of limbs of upper diaphragm spring 8 and counteract an upward deflection of the oscillating mass, are provided on upper yoke 29.

Upper yoke 29 also supports a cylinder 33 in which a piston connected to oscillating mass 24 by means of a piston rod 10, not shown in the figure, is able to move back and forth. Since oscillating mass 24 is guided exactly linearly by the two diaphragm springs 8, piston rod 12, and with it the piston supported by it, cannot deviate transversely to the direction of movement and grinding of the piston against the inner wall of cylinder 33 can be avoided. Due to the movement of the piston on the inner wall of cylinder 33, fluid is sucked in through a suction connection 24 of cylinder 323, is compressed and is again ejected via a pressure connection 35.

When oscillating mass 24 is located at one of the points of inversion on its trajectory, its entire kinetic energy is stored in the diaphragm springs 8 and the helical springs 31 in the form of deformation energy, the distribution of the energy among the spring types depending on their respective spring constants. The diaphragm springs may therefore be made very thin and easily deformable so that no material fatigue occurs even during protracted operation. For the energy which the diaphragm springs are unable to store due to insufficient stiffness may be absorbed by suitably dimensioned helical springs 31.

Moreover, compressors with different throughputs can be achieved with the same model of diaphragm spring if the diaphragm springs are each combined with helical springs with different spring constants, resulting in different natural frequencies of the oscillating system.

It is also conceivable to render the natural frequency of a drive unit adjustable by mounting helical springs 31 displaceably on yokes 29, 30. The closer the region of limbs 14 touched by head pieces 32 of helical springs 31 is to central region 16 of diaphragm springs 8, the stiffer will be the entire system, consisting of the diaphragm spring and helical springs, and the higher will be the natural frequency of the resultant drive unit.

In the extreme case it is possible to replace the two helical springs 31 of each yoke 29, 30 by a single helical spring which touches central region 16 directly.

FIG. 3 shows a modification of diaphragm spring 8 from FIG. 3, which can be used in its stead in the compressor shown in FIG. 4. In the case of the diaphragm spring shown in FIG. 5, outer protective frame 13 is omitted and instead only the three right and two left limbs 14 are connected at their ends facing away from central region 16 by a material strip 34. Here the limbs are wider, given the same external dimensions of the diaphragm spring, and are therefore stiffer than those of the spring in FIG. 2. The mode of operation is no different to that of the diaphragm spring shown in FIG. 3. 

1-12. (canceled)
 13. A drive unit for a linear compressor comprising a frame and an oscillating body mounted in the frame by means of at least one diaphragm spring and moving back and forth, the diaphragm spring having a plurality of limbs which engage the frame with a first end and engage the oscillating body with a second end, wherein each limb includes a readjusting spring which counteracts a deformation of the limb.
 14. The drive unit according to claim 13, wherein the limbs have a curved path between the two ends.
 15. The drive unit according to claim 14, wherein each arm has two sections curved in different directions.
 16. The drive unit according to claim 13, further comprising a second diaphragm spring, the first and second diaphragm springs engaging regions of the oscillating body that are spaced in the direction of the oscillating movement.
 17. The drive unit according to claim 13, wherein the limbs of the same diaphragm spring are integrally joined together at their ends engaging on the oscillating body.
 18. The drive unit according to claim 13, wherein the limbs of the same diaphragm spring are integrally joined together at their ends engaging on the frame.
 19. The drive unit according to claim 18, wherein the ends engaging on the frame are connected by a frame integral with the limbs.
 20. The drive unit according to claim 13, wherein the stiffness of the diaphragm spring is lower in the direction of deformation than that of the readjusting spring.
 21. The drive unit according to claim 13, wherein an effective spring constant of the combination of diaphragm spring and readjusting spring is adjustable.
 22. The drive unit according to claim 13, wherein the readjusting spring includes a helical spring.
 23. The drive unit according to claim 13, wherein the mass of the oscillating body is greater than the mass of all the springs.
 24. A linear compressor comprising: a working chamber; a piston that is able to move back and forth in the working chamber for compressing a working fluid; and a drive unit coupled to the piston for driving the back and forth movement and comprising a frame and an oscillating body mounted in the frame by means of at least one diaphragm spring and moving back and forth, the diaphragm spring having a plurality of limbs which engage the frame with a first end and engage the oscillating body with a second end, wherein each limb includes a readjusting spring which counteracts a deformation of the limb. 